The present invention may be used to improve the quality of audio signals obtained from upmixing; however, the present invention may be used advantageously with essentially any application that requires one or more audio signals representing a diffuse sound field. More particular mention is made of upmixing applications in the following description.
A process known as upmixing derives some number M of audio signal channels from a smaller number N of audio signal channels. For example, audio signals for five channels designated as left (L), right (R), center (C), left-surround (LS) and right-surround (RS) can be obtained by upmixing audio signals for two input channels designated here as left-input (Li) and right input (Ri). One example of an upmixing device is the Dolby® Pro Logic® II decoder described in Gundry, “A New Active Matrix Decoder for Surround Sound,” 19th AES Conference, May 2001. An upmixer that uses this particular technology analyzes the phase and amplitude of two input signal channels to determine how the sound field they represent is intended to convey directional impressions to a listener. Depending on the desired artistic effect of the input audio signals, the upmixer should be capable of generating output signals for five channels to provide the listener with the sensation of one or more aural components having apparent directions within an enveloping diffuse sound field having no apparent direction. The present invention is directed toward generating output audio signals for one or more channels that can create through one or more acoustic transducers a diffuse sound field with higher quality.
Audio signals that are intended to represent a diffuse sound field should create an impression in a listener that sound is emanating from many if not all directions around the listener. This effect is opposite to the well-known phenomenon of creating a phantom image or apparent direction of sound between two loud speakers by reproducing the same audio signal through each of those loud speakers. A high-quality diffuse sound field typically cannot be created by reproducing the same audio signal through multiple loud speakers located around a listener. The resulting sound field has widely varying amplitude at different listening locations, often changing by large amounts for very small changes in location. It is not uncommon that certain positions within the listening area seem devoid of sound for one ear but not the other. The resulting sound field seems artificial.